The present invention relates to a piston-type positive-displacement pump and also to a valve arrangement which can be used, for instance, as an inlet valve in such pumps.
European Patent Application No. 89850427.9 discloses piston-type positive-displacement pumps, some embodiments of which are provided with flap valves and other embodiments of which are provided with mechanically operated pinch valves for blocking and unblocking the flow of fluid between an inlet and a pump chamber and in certain cases also between the pump chamber and an outlet.
An important feature of the pumps disclosed in the aforesaid patent application is that the inlet communicates with the pump chamber through a flow passage in the form of a gap which extends around the full circumference of the pump chamber, or at least over a major part of the circumference.
In those embodiments where the outlet is positioned centrally in the pump chamber, the pump can be readily constructed so that the flow passage will extend completely around the pump chamber, such that the passage will have a maximum circumferential length for a given diameter of the pump chamber and can thus be given a maximum cross-section area. Connected with the flow passage, immediately outside it, is a supply or reservoir chamber, which also extends completely around the pump chamber. Some of these embodiments include flap valves, while others include pinch valves which are operated positively by mechanical actuating means.
In the case of other embodiments, a small part of the pump-chamber circumference is used to accommodate an outlet connection from the pump chamber, with the circumferential ends of the flow passage positioned adjacent the outlet connection on both sides thereof. In these emodiments, the flow of fluid through the flow passage is controlled by means of flap valves.
In all of the emodiments illustrated in the aforesaid patent application, those parts of the pump which come into contact with the pumped fluid are formed by a container which is made of a flexible material (plastic foil or plastic film) and which comprises an inlet connection, a supply or reservoir chamber into which the inlet connection opens, a pump chamber, a flow passage extending between the two chambers, and an outlet connection, which extends from the pump chamber and which may include an equalizing chamber. The container is formed from two superimposed plastic-foil sheets which are heat-sealed together to form the just-mentioned parts.
When a pump of the kind described in the aforesaid patent application is to be used as a metering or volumetric batching pump with which high demands are placed on the exactness of the volume of fluid metered or batched by the pump, as is the case with liquid-packaging machines, for instance, it is preferred to use mechanically and positively controlled pinch valves, since flap valves are not able to close with sufficient precision. The use of positively controlled pinch valves causes no particular problems in those instances where the flow passage extends around the whole of the pump-chamber circumference, since in such cases the container walls may part from one another along a closed curve (circle) to open the flow passage. On the other hand, a problem is created when the flow passage is broken or interrupted for accommodating the outlet connection, since the container walls are heat-sealed together or held together in some other way at the ends of the flow passage.